1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a decoding apparatus and method of a terminal.
2. Description of the Related Art
Many wireless communication technologies have been proposed for high-speed mobile communications. Among them, Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiplexing Access (OFDMA) are considered as the most powerful next generation wireless communication technology. OFDM/OFDMA technologies are expected to come into commercial use in most wireless communication technologies around 2010. Institute of Electrical and Electronics Engineers (IEEE) 802.16 Wireless Metropolitan Area Network (WMAN), called 3.5th (3.5G) generation technology, adopts the OFDM/OFDMA technologies as standard specification.
OFDM scheme transmits data using a multi-carrier scheme. Specifically, the OFDM scheme is one of multi-carrier modulation schemes that parallel-converts serial symbol streams and modulates the parallel symbol streams into a plurality of sub-carriers having mutual orthogonality, i.e., a plurality of sub-carrier channels.
The OFDM scheme is similar to the Frequency Division Multiplexing (FDM) scheme, but can obtain optimal transmission efficiency during a high-speed data transmission by maintaining orthogonality between the sub-carriers. In addition, because the OFDM scheme overlaps the frequency spectrum, it can use the frequency band efficiently and is robust against frequency selective fading and multipath fading. Moreover, the OFDM scheme reduces the effect of intersymbol interference (ISI) by using a guard interval, simplifies the structure of an equalizer, and is robust against impulse noise. Thus, the OFDM scheme is widely used in the communication systems.
While OFDM allocates the entire resource, i.e., the sub-carriers to a single user, the OFDMA scheme allocates the sub-carriers to multi users. The information about the resource allocation is transmitted through a MAP message to which a high modulation and coding scheme (MCS) level is applied so that all subscriber terminals can commonly receive the information. Each user determines the location of the allocated resource and data control information by decoding the MAP message and transmits uplink data or downlink data.
Referring to FIG. 1, a carrier to interference and noise ratio (CINR) estimator 101 estimates a CINR value through a preamble of a downlink frame or a pilot signal. The CINR represents a channel quality (condition). An RX physical layer (PHY) buffer 103 stores downlink bursts and outputs them to a convolutional turbo code (CTC) decoder 105. CTC decoder 105 decodes the received bursts according to the MCS level and outputs the decoded bursts to a low medium access control (LMAC) buffer 107. LMAC buffer 107 stores the decoded bursts inputted from CTC decoder 105.
The MAP message includes a downlink-MAP (DL-MAP) message and an uplink-MAP (UL-MAP) message. The DL-MAP message includes an MCS level index of an allocated radio resource block, i.e., a downlink interval usage code (DIUC) and number of repetitions of a repetition code (hereinafter, referred to as a repetition), and location information of the radio resource block. In some cases, the DL-MAP further includes a connection identifier (CID) of a subscriber terminal that will use the allocated radio resource block. The UL-MAP message includes an MCS level index of the allocated radio resource block, i.e., uplink interval usage code (UIUC) and repetition, location information of the radio resource block, and CID of the subscriber terminal that will use the allocated radio resource block.
The CID is an MAC layer address to identify the connection and is called a basic CID in IEEE 802.16. In the downlink direction, the terminal discerns the incoming information by referring to the allocated CID. Thus, the terminal can receive the burst by decoding it to a corresponding MCS level. In the uplink direction, by referring to the inherent CID defined by the base station the terminal can transmit data through an interval defined by the base station.
However, there may exist bursts having no CID information within the frame. In this case, the terminal cannot obtain the CID information of the corresponding burst from the DL-MAP message. Therefore, the terminal decodes the bursts having no CID and then discerns its own data based on transport CID information loaded on a generic MAC header of a MAC packet data unit (PDU) within the corresponding burst. That is, the terminal tries to decode the bursts having no CID at all MAC levels, regardless of its own channel state or CINR representing the channel state.
When the CINR value is low, the decoding result may not satisfy a target packet error rate (PER), and this value may be useless in the upper layer. The target PER represents a target performance rate. The base station allocates the resource according to the channel conditions. Therefore, when the channel condition is poor, there is a low possibility that the base station transmits/receives data through a high MCS level. Consequently, the terminal unnecessarily operates the decoder for decoding the burst, thus wasting the power of the terminal with limited battery power.